1. Field of the Invention
The present invention relates to electronic printing form production utilizing engraving devices and scanners, and is more particularly concerned with a method and a circuit arrangement for contrast intensification in the reproduction of masters.
2. Description of the Prior Art
In electronic printing form production utilizing engraving devices and scanners, a master to be reproduced is scanned point-wise and line-wise with an opto-electronic scanning element and the image signal thereby obtained is corrected in accordance with the requirements of the reproduction process. The corrected image signal controls a recording element which undertakes the reproduction of the master on a recording medium.
Given printing form engraving devices, the production of the raster points occurs with an engraving stylus of an electro-magnetic engraving element or with the energy beam of an energy beam generator as the recording element. Given scanners, the recording element is a light source for film exposure.
In most cases, the master to be reproduced is a combination consisting of continuous-tone or half-tone images and of written characters or line elements or patterns of graphic illustrations.
As early as the photographic production of a film master, the contrast, particularly in the fine details, is reduced in comparison to the original due to unsharpness in the film layers, as well as due to enlargement for transfer. Added thereto is the limitation of resolution of the opto-electronic scanning element by scatter light and by unsharpness of the scanning lens, a further contrast reduction occurring in the reproduction as a result thereof which the human eye perceives as unsharpness or lack of definition.
Therefore, it is necessary to regain the reduced contrast or, respectively, the reduced image sharpness in the printing form production or, for editorial reasons, of even increasing the contrast in comparison to the original.
For the purpose of contrast or, respectively, sharpness intensification, it is already known from U.S. Pat. No. 2,691,696, fully incorporated herein by this reference, to scan not only the current image point with an image point diaphragm but, rather, to also scan its surrounding field with a correspondingly larger outer field diaphragm, to form the differential signal of the outer field signal and the image signal, and to superimpose the differential signal on the image signal to a selectable degree as a correction signal. This measure, which corresponds to unsharp masking in the photographic reproduction technique, leads to an improvement of the detail contrast or, respectively, of the image sharpness at jumps of gradation or at contours since, in the direct environment of a gradation jump, a dark image passage is recorded darker and a light image passage is recorded lighter than at some distance from the gradation jump. In the known solution, the amplitude of the correction signal is dependent on the size of the respective gradation jump so that, normally, line elements and character contours (high gradation jumps) are greatly corrected but, in contrast thereto, contours in continuous-tone or half-tone images (small gradation jumps) are only slightly corrected.
In practice, however, it is a matter, particularly, of boosting the detail contrast in continuous-tone or half-tone areas since line elements and character contours are reproduced sharper anyway. In order to be able to attain the desired effect in continuous-tone or half-tone areas at all, the available correction signal must be superimposed, full strength, on the image signal in the cited process. In this case, however, hard contours arise, particularly at highlights, and disturbing, white margins or edges arise around black characters, these being particularly noticeable when, for example, black print is placed in a gray scale. Therefore, the disadvantage of the known technique is that it can be respectively optimized only to continuous-tone respectively half-tone images or to line elements.
Attempts have already been made to improve the known method by distortion, amplitude limitation or by an asymmetrical treatment of the correction signal, as well as by influencing the correction signal as a function of a shift mask (German Letters Patent No. 9 49 443, German Letters Patent No. 10 39 842 and German Letters Patent No. 22 26 990), however, there are then still difficulties in practice of avoiding such margins or edges or of at least reducing the same in such a manner that they are no longer disturbing.